Kinetics

Kinetics

by Jennifer Haycraft

​Kinetics (Chapter 14)

​Studies the rate at which a chemical process occurs.

Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs).

​Factors that Affect Rate

1. Molecules must come in contact with each other with the right energy and correct orientation.

2. ​The more homogeneous the mixture the faster the rate.

3. ​If concentration increases then rate increases

​More Factors

​4. Temperature - Greater temperature, molecules move faster, more collisions, with more energy.

​5. Presence of a catalyst - speeds up the reaction by changing the mechanism, is not consumed during the reaction.

​Enzymes are biological cataylsts

​Reaction Rates

​Rates can be determined by monitoring the change in concentration of the reactants or the products as a function of time.

Multiple Choice

How does increasing the partial pressures of the reactive components of a gaseous mixture affect the rate at which the components react with one another?

• The effect of increasing the partial pressures of the reactive components of a gaseous mixture depends on which side of the chemical equation has the most gas molecules.

• Increasing the partial pressures of the reactive components of a gaseous mixture has no effect on the rate of reaction if each reactant pressure is increased by the same amount.

• Increasing the partial pressures of the reactive components of a gaseous mixture increases the rate of reaction.

• Increasing the partial pressures of the reactive components of a gaseous mixture decreases the rate of reaction.

​Reaction Rates

Open Ended

From the data given, calculate the average rate at which A disappears over the time interval from 20 s to 40 s.

$$Average\ Rate\ =\ -\frac{\Delta\left[A\right]}{\Delta t}$$  

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Open Ended

Calculate the average ratee of the appearance of B over the time interval from 0 to 40 s. 

​Rates

​The average rate decreases as the reaction proceeds.

​This is because as the reaction goes forward, there are fewer collisions between molecules.

​Rxn rates

Look at the plot of concentration vs. time, the slope of a line tangent to the curve at any point is the instantaneous rate at that time.

​All rxns slow down over time. It is best to find the instanteous rate the beginning.

Open Ended

Calculate the instantaneous rate of disappearance of  $$C_4H_9Cl\ at\ t\ =\ 0$$   (the initial rate).

$$Rate\ =\ -\ \frac{\Delta\left[C_4H_9Cl\right]}{\Delta\ t}$$  

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​Reaction Rates and Stoichiometry

Open Ended

How is the rate at which ozone disappears related to the rate at which oxygen appears in the reaction  $$2\ O_3\left(g\right)\ \rightarrow\ 3\ O_2\ \left(g\right)$$  ?

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Open Ended

If the rate at which  $$O_2$$  appears,  $$\Delta\left[O_2\right]\ =\ \Delta t,\ is\ 6.0\ x\ 10^{-5}\ \frac{M}{s}$$   at a particulat instant, at what rate is  $$O_3$$   disappearing at this same time,  $$-\ \Delta\ \left[O_3\right]\ =\ \Delta t$$  ?

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